For quite a number of years the technology of hot swapping of electric components has been known in the computer and the telecommunication fields.
Hot swapping means adding or removing electric components into an existing computer or telecommunication architecture without having to perform a power shut-down of the whole architecture first.
In the network technology fields, this technology provides the possibility of adding or removing network adapters to or from a backplane, while the backplane is still delivering power to other components in the network. Likewise, hard drive units in a redundant backup system, such as a RAID (Redundant Array of Independent Disks), can be plugged in or taken out from the server machine, while the server is still running.
The hot swap technology is being used in the computer field in order to insert different kinds of adapters onto a motherboard on a computer, while it is still being provided with power or as external devices plugged into USB (Universal Serial Bus)- or FireWire-ports belonging to a computer.
Hot-swapping in telecommunication applications allows for example insertion or removal of line cards to or from a backplane in a base station, while the backplane is still delivering power to other line cards connected to it. Thus system upgrades, maintenance and repair are performed much faster with minimum disturbance to users of the telecommunication network.
One common problem when hot-swapping units into an existing computer- or telecommunications system is the occurrence of large inrush currents, which can exceed the operating current of the power supply and thus damage either the computer- or telecommunication system or the component itself, or both.
The reason for this put simply is that the component to be added to the system often presents a high capacitive load to the power source of the system and large capacitors need time to be loaded. However, during the sudden current surge caused by the switch-on of the power source circuit, the large capacitor acts as a short circuit, thus leading to a large inrush current going through the circuit of the component to be added to the computer- or telecommunication system.
Similar phenomena are observed when such modules are removed from the computer- or telecommunication system.
The first and easiest way of dealing with inrush currents are the use of discrete components in the form of thermistors whose resistance is current dependent and increasing or decreasing with increasing current, so called PCTs and NTCs. Thus, during switch-on procedure of the power supply, a thermistor is warmed up by the current flowing through it and slowly allows the current to rise, when a large capacitive load is added to the power supply circuit. In this way, the initial rapid current rise due to the addition of the capacitive load is slowed down.
A disadvantage of the thermistor solution is its inherent slow response to current transients and might not be desirable in an environment, where modules are frequently inserted or removed from for example a telecommunication system.
Thus, in addition to thermistors, rapid changes have to be responded to by fuses or fault protection devices, where the fuse add a voltage drop to the power path, which is generally not desired in these applications. Also, the thermistors themselves add a voltage drop to the power path.
A different solution still using discrete components is the use of discrete MOSFETS, which provide low drain-to-source resistance Rds(on) and act almost as an ideal switch.
MOSFETs require low voltages to operate and can be switched on or off rapidly in order to respond rapidly to voltage changes.
The disadvantage of discrete MOSFETs for protection against inrush currents is the additional circuitry in the form of resistors and capacitors necessary to control the current rise time and different fault conditions, such as overcurrent.
Normally, such discrete MOSFETs are expensive and difficult to optimize according to the respective applications.
Also, discrete MOSFETs have a parasitic diode connected from the drain to the source, which can lead to current backflow, when the output voltage on the device is higher than the input voltage.
The object of the present invention is to rectify some of the disadvantages with known technology described above.